Hi J.,
I had my daughter 2 weeks after my son turned 2, so I know what you are going through.
The way we did it, is that that first I started pointing out babies that we saw while doing our regular things (eg, grocery shopping, going to church) and saying something like "Aww, look how cute that baby is. Don't you want one?" We also started watching tv shows about babies (I looked on TLC, PBS, Discovery, etc). When I started to show, I told him that there was a baby growing in there. I also MADE SURE to let him know that it was his baby too. We bought the twin baby doll set (one for me, one for him). During play time, I would just hold the doll and pretend it was my baby. After a while, he started to show interest in his doll (don't be discouraged it he throws it or bashes it--he is a boy :-) ). When we were able to feel the baby kick (on the outside), I made sure to let him feel here (all the while, letting him know that it was his baby in there). We also made sure to say what a good big brother he was going to be. I would ask him if he was going to be my good helper and he would excitedly answer "Yes." When it got close to the delivery date, we really started talking it up..."Emily will be here next week, aren't you excited? She will be so happy to see her big brother." Since I had a scheduled c-section, it was easy to plan for it. The day before she was born, I went to the pet store and bought him a fish (he loves fish, dolphins, etc). I asked my mother to make sure the fish bowl was set up for him when he got back from meeting her at the hospital.
When the day actually arrived, it was so prescious. As soon as he came into our room, we asked him..."Carson, guess who is here." A big grin broke out on his face and he said "Emily." When my mother brought her over to him, he asked to hold her. With the help of my mother, he sat in the chair and held her. I literally wouldn't let got for 10 minutes.
To this day, he calls her his baby (she is 18 months old). It is unbelievable how much he takes care of her.
The only thing I would want to warn you about it be sure not to make him feel second in line. Even though I put aside an hour every night for Carson and mommy time, the rest of the day, whenever he asked for something, I would say..."Sure honey, after I am finished (diapering, feeding, etc) Emily." I didn't notice it until he started having tantrums when she was about 4 months old. When I started to really look at what I could be doing to make him angry, I came up with that. Once I started to remedy it (Emily wouldn't care if she stayed in her diapers for another 3 minutes, or if we delayed her bath time by 10 minutes) the tantrums went away.
I know this was REALLY long, but I wanted to give you all the advice I had.
Good luck!
T.